Few weeks back I have noticed that nas540 have been busy more the usual and by looking in to storage status its been busy with disk resync.
I did not gave it a second though and waited until the next morning. On first glance everything was ok until I opened plex client on my smart TV
and realised that 30% of my episode from fav shows are gone.

Storage status is still saying that disks are ok but now I am very sceptical where I can live with out tv shows I CANT live with lost documents and photos.
I checked forum for disk tools that I could use and installed FFP only to hit a wall with checksum error:

badkernel wrote:Is there a choice in testdisk that can actually show status of disk in RAID ?

What do you mean?

What I ment was is there obvious choice (option aka flag) with testdisk that can show previous errors on RAID?
I mean all 4 disk are fine only files are missing and logs available to me do not mention any kind of foul play.

But a raid problem should not be able to remove a part of the files. The array is up or down, there is nothing in between. (Of course there is also 'degraded', but in that case the filesystem is still intact. Only the redundancy is lost)

You might have had a filesystem error, in which case you might be able to find your files in /i-data/sysvol/lost+found/ , but the most probable cause is that you deleted the files yourself, somehow.